Cepaeanemoralis, commonly known as the Grove Snail, has been the subject of scientific study since the time of the renowned taxonomist Linnaeus. C. nemoralis is a cross-fertilizing hermaphroditic mollusc that is able to tolerate a wide range of habitats. These snails display polymorphism – different phenotypic forms among the same species. C. nemoralis shows polymorphism for shell colour, the presence, number and appearance of bands on its shell; the main colour classes are pink, yellow and brown and there are between 0 to 5 bands on each shell. Shell polymorphisms within the different populations of this species directly reflect the distribution and frequency of the different genotypes present, allowing genotypic analysis. Due to its slow movement speed and an average lifespan of 2.3 years1, a population of C. nemoralis is limited to a small geographical area, thus making the counting of polymorphs of the population possible. This combined with a 4 year generation time means that genetic changes within different populations remain stable over many generations. In previous studies into polymorphisms of C. nemoralis, it is suggested that different polymorphic forms appear due to different allele frequencies within the populations, thus causing vast phenotypic differences over a small area2. With previous studies into this subject in mind, further research into polymorphic frequency has been done in order to ascertain whether elevation or habitat type is a contributing factor in polymorphism of C. nemoralis. In the deduction of this, the effects of genetic drift, natural selection and pseudoreplication must be taken into account. Genetic drift describes the process by which allele frequencies change over time due to the effects of random sampling. Drift takes places due to a finite population size and has its greatest effect in smaller population sizes. Genetic drift occurs in all populations and any selection must occur against this background of drift. Natural selection is the process by which individuals of a population have different levels of fitness due to their individual genotypes, thus increasing the likely hood of more fit individuals surviving selection pressures, such as predators or finite resources, due to their greater ability to adapt to the situation compared to members of the same population within that situation; this increases the frequency of the more ‘fit’ alleles that those individuals possess due to their increased number of progeny3.
Introduction Word Count = 384 words
Preliminary sampling scheme: The original sampling scheme was to test three bushes at the same altitude and the grass areas in between them. This aimed to repeat readings for the two environmental conditions. However, on viewing the site it was possible to improve this plan.
Revised sampling scheme: It was resolved that 3 bush areas and 2 grass areas would be studied at the same altitude. This was to determine whether the environment affected phenotype (and therefore genotype). The first 2 bushes were approximately 5m apart; this ensured gene flow was likely to occur between the two populations. The third bush was approximately 30m away; this ensured gene flow with either of the other bushes was highly unlikely, given the typical lifestyle of the snails. The grass areas between the bushes were also studied. By reducing the number of sites studied, it would be possible to obtain information about more snails for each site, to get a fuller picture of what is happening.
Once the areas were chosen, snails were identified to be of the correct species. Information about the colour and banding of the shells was collected for the first 30 snails found in each site. This information was recorded in a table using a tally. From this data it may be possible to infer whether environment has any effect on phenotype, and whether this effect is due to natural selection, genetic drift or both.
Introduction
Cepaea nemoralis, commonly known as the Grove Snail, has been the subject of scientific study since the time of the renowned taxonomist Linnaeus. C. nemoralis is a cross-fertilizing hermaphroditic mollusc that is able to tolerate a wide range of habitats. These snails display polymorphism – different phenotypic forms among the same species. C. nemoralis shows polymorphism for shell colour, the presence, number and appearance of bands on its shell; the main colour classes are pink, yellow and brown and there are between 0 to 5 bands on each shell. Shell polymorphisms within the different populations of this species directly reflect the distribution and frequency of the different genotypes present, allowing genotypic analysis. Due to its slow movement speed and an average lifespan of 2.3 years1, a population of C. nemoralis is limited to a small geographical area, thus making the counting of polymorphs of the population possible. This combined with a 4 year generation time means that genetic changes within different populations remain stable over many generations. In previous studies into polymorphisms of C. nemoralis, it is suggested that different polymorphic forms appear due to different allele frequencies within the populations, thus causing vast phenotypic differences over a small area2.With previous studies into this subject in mind, further research into polymorphic frequency has been done in order to ascertain whether elevation or habitat type is a contributing factor in polymorphism of C. nemoralis. In the deduction of this, the effects of genetic drift, natural selection and pseudoreplication must be taken into account. Genetic drift describes the process by which allele frequencies change over time due to the effects of random sampling. Drift takes places due to a finite population size and has its greatest effect in smaller population sizes. Genetic drift occurs in all populations and any selection must occur against this background of drift. Natural selection is the process by which individuals of a population have different levels of fitness due to their individual genotypes, thus increasing the likely hood of more fit individuals surviving selection pressures, such as predators or finite resources, due to their greater ability to adapt to the situation compared to members of the same population within that situation; this increases the frequency of the more ‘fit’ alleles that those individuals possess due to their increased number of progeny3.
Introduction Word Count = 384 words
Preliminary sampling scheme:
The original sampling scheme was to test three bushes at the same altitude and the grass areas in between them. This aimed to repeat readings for the two environmental conditions. However, on viewing the site it was possible to improve this plan.
Revised sampling scheme:
It was resolved that 3 bush areas and 2 grass areas would be studied at the same altitude. This was to determine whether the environment affected phenotype (and therefore genotype). The first 2 bushes were approximately 5m apart; this ensured gene flow was likely to occur between the two populations. The third bush was approximately 30m away; this ensured gene flow with either of the other bushes was highly unlikely, given the typical lifestyle of the snails. The grass areas between the bushes were also studied. By reducing the number of sites studied, it would be possible to obtain information about more snails for each site, to get a fuller picture of what is happening.
Once the areas were chosen, snails were identified to be of the correct species. Information about the colour and banding of the shells was collected for the first 30 snails found in each site. This information was recorded in a table using a tally. From this data it may be possible to infer whether environment has any effect on phenotype, and whether this effect is due to natural selection, genetic drift or both.